Biogas
Biogas is a mix of gases, mostly methane, made when tiny living things break down waste with no air.
Think of it this way
A sealed jar holds food scraps and water. Tiny germs inside eat the scraps and release gas. The gas fills the top of the jar. That trapped gas is biogas — mostly methane.
Explaining biogas by grade level
When cow manure sits in a sealed jar, germs eat the waste. They make a gas that can burn like a stove flame. The gas builds up in the jar over time. A warm jar makes more gas than a cold one.